Chromium (Cr) is a redox-sensitive element and because Cr isotopes are fractionated by redox and/or biological processes, the Cr isotopic composition (δ⁵³Cr) of ancient marine sediments has the potential to infer changes in past seawater oxygenation or biological productivity. Although growing evidence suggests that Cr isotopes could serve as a proxy for reconstructing changes in the biological carbon pump over time, their application remains limited by a lack of systematic observations from both the modern ocean and the geological record.
In this presentation, we will first introduce recent analytical and instrumental advances in Cr isotope measurements developed in collaboration with the Southern Marine Laboratory. We will then present observations from an oyster farm in Huangmaohai Estuary, west of Zhuhai, where paired dissolved and particulate Cr isotope data reveal the preferential sequestration of isotopically light Cr into the particulate phase. Finally, we will discuss a systematic decrease in sedimentary δ⁵³Cr values during the Early Eocene Climatic Optimum, an interval characterized by enhanced marine productivity. Together, these modern and geological observations provide new insights into the oceanic processes controlling Cr isotope fractionation and therefore the efficacy of δ⁵³Cr as productivity tracer.

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